Nitrogen-containing compounds are used as structural components of pharmaceuticals and agrochemicals due to their high biological activity. Among these compounds, pyridine bases are produced in by far the largest quantity and are used in various applications such herbicides, insecticides, pharmaceuticals and adhesives.
The base synthesis of pyridine and its derivatives is well known. The process generally involves reacting aldehydes and/or ketones with ammonia in the gas phase using a heterogeneous catalyst either in a fixed bed or fluidized bed reactor at temperatures ranging from about 400° C. to about 450° C. The reaction generates coke and the catalyst has to be regenerated with air. The use of a fluidized bed provides a useful continuous regeneration system.
Catalysts used in pyridine base synthesis reactions have varied from alumina either alone or as a support, to amorphous silica alumina (see e.g., U.S. Pat. Nos. 3,272,825; 3,946,020; and 4,089,863) and/or metal substituted silica alumina (see e.g., R. A. Sheldon and H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis, Ed, Wiley p 277). However, in recent years, the focus has shifted to the use of so-called “shape selective” zeolite, e.g. aluminosilicates of a definite crystal structure and pore size and characteristic, based catalyst systems. A major breakthrough in this area came with the use of the zeolite ZSM-5, also called “MFI”, which showed improved pyridine yields in the pyridine beta reaction due to the shape selectivity offered by the size and two dimensional pore channels of the zeolite (see e.g., U.S. Pat. Nos. 4,220,783; and 4,675,410). Improved pyridine yields were found in ZSM-5 zeolites having silica/alumina ratios between 150-400 (see e.g., R. A. Sheldon, H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis, Ed: Wiley p 277). Further improvements were seen in the development of metal substituted ZSM-5 zeolites. For example, zeolites ion-exchanged with thallium, lead or cobalt showed increased yields of pyridine bases (see e.g., U.S. Pat. No. 4,810,794). Other metal substituted zeolites used in pyridine catalysts have included ZSM-5 zeolites modified with one or more metal ions of zinc, tin or tungsten. (see for example, U.S. Pat. No. 5,218,122).
Because it uses inexpensive and widely available raw materials, base synthesis of pyridine continues to provide good prospect to meet the growing demands for pyridine and its alkyl derivatives. However, there remains a need for improved processes and catalysts which are useful to enhance product yields of pyridine and alkyl pyridine derivatives during base synthesis reactions.